84 M491 Ride Height Questions

[Quasimoto]

Hello, we recently acquired a 1984 M491 Coupe, and have a few questions about the ride height. (Visually, the car appears to be setting a little high in the front.)

Here are my measurements. (Stock wheels w/ 205/55-16 & 225/50-16)

136mm - Front ride height as measured per the Workshop Manual
-25mm - Rear
(Workshop manual seems to call for 108/16 respectively)


I also measured the fender lips as I have read threads on here that reference this...although not sure if it applies to a car with flares.
26" - Front fender lip
25" - Rear fender lip


Questions
Are these baseline numbers I am referencing correct for an M491?

If so, does this mean that the car has been lowered 28mm Front and 41mm rear?

[Quasimoto]

Attempting to deal with the rear height, but the spring plate clamp bolt is seriously frozen.

Any tips on the best method for freeing these.up?

[BSNMOE]

Be careful if using a socket with breaker bar. My friend had the socket break off, flew up and put ding in the fender. Maybe heat the nut then put damp rug over socket.

[Bill Verburg]

Originally Posted by Quasimoto

Hello, we recently acquired a 1984 M491 Coupe, and have a few questions about the ride height. (Visually, the car appears to be setting a little high in the front.)

Here are my measurements. (Stock wheels w/ 205/55-16 & 225/50-16)

136mm - Front ride height as measured per the Workshop Manual
-25mm - Rear
(Workshop manual seems to call for 108/16 respectively)


I also measured the fender lips as I have read threads on here that reference this...although not sure if it applies to a car with flares.
26" - Front fender lip
25" - Rear fender lip


Questions
Are these baseline numbers I am referencing correct for an M491?

If so, does this mean that the car has been lowered 28mm Front and 41mm rear?

Factory spec for RoW is 18+/-5mm front and 16+/-5mm rear

I seem to recall that is ~25.5" fender height at both ends and that this should give a slight nose down.

most seem to settle on a fender height ~25"/25" but many also go lower 24.5"/24.5" isn't uncommon. At the lower ride heights you do want rack spacers.

[Quasimoto]

Originally Posted by BSNMOE

Be careful if using a socket with breaker bar. My friend had the socket break off, flew up and put ding in the fender. Maybe heat the nut then put damp rug over socket.

Sounds like good advice, thanks!

Originally Posted by Bill Verburg

Factory spec for RoW is 18+/-5mm front and 16+/-5mm rear

I seem to recall that is ~25.5" fender height at both ends and that this should give a slight nose down.

most seem to settle on a fender height ~25"/25" but many also go lower 24.5"/24.5" isn't uncommon. At the lower ride heights you do want rack spacers.

Thanks Bill, was hoping you would chime in...
Is it possible that this car has settled this much in the rear or is it more likely that someone lowered it?

My fender dimensions are very close to those numbers even though the ride height is set significantly lower. Not sure what is going on there...need to do some more reading/investigation on the car.


What I'm after is to correct the ride height and get rid of the nose up / tail down look at the same time.

Poopy bolts are giving me trouble right now, but I'll get that sorted...

[Bill Verburg]

Originally Posted by Quasimoto

...
Is it possible that this car has settled this much in the rear or is it more likely that someone lowered it?

My fender dimensions are very close to those numbers even though the ride height is set significantly lower. Not sure what is going on there...need to do some more reading/investigation on the car.


What I'm after is to correct the ride height and get rid of the nose up / tail down look at the same time.

Poopy bolts are giving me trouble right now, but I'll get that sorted...

Bilstein shocks will raise the car a bit, don't know what you have other than that they don't settle unless something is broken.

The #s I gave are from memory, probably close but you have to go by what you measure on your car. This is a game of mm's, going from ~25.5" to 24.5" will change a hypothetical 108+/- to 133+/-

You definitely ought to want a bit of a nose down attitude, another set of often thrown around figures is 24.5"/ 24" or 25" /24.5" ie .5" lower at the rear fender, On my car this seems to give a pretty level stance. Which is why 25/25 or 24.5/24.5 seems a bit more apt to me.

The thing to remember is that fender heights are notoriously unreliable i.e. repeatable on different cars.

[Quasimoto]

Originally Posted by Bill Verburg

This is a game of mm's, going from ~25.5" to 24.5" will change a hypothetical 108+/- to 133+/-

Makes sense, hadn't considered that yet. Once I get the bolts free, I'll tweak and measure again.

I do have access to wheel scales, so that may be the next step after that.

Here's a side pic of the car, not sure if it really captures the "tail-down" attitude the car currently has though...

[Bill Verburg]

Originally Posted by Quasimoto

Makes sense, hadn't considered that yet. Once I get the bolts free, I'll tweak and measure again.

I do have access to wheel scales, so that may be the next step after that.

Here's a side pic of the car, not sure if it really captures the "tail-down" attitude the car currently has though...

It looks to me that both ends can come down

[76FJ55]

Bill, seems you lost the center "0" in your first post for front ride height.

Quasimoto, make sure you are loosening the nut side of the adjustment bolts not the bolt side. the eccentrics bottom in the slot so it you are trying to turn the bolt you may just be forcing it up against the end of the slot.

[Bill Verburg]

Originally Posted by 76FJ55

Bill, seems you lost the center "0" in your first post for front ride height.

Quasimoto, make sure you are loosening the nut side of the adjustment bolts not the bolt side. the eccentrics bottom in the slot so it you are trying to turn the bolt you may just be forcing it up against the end of the slot.

right you are
108+/-5mm is the front spec

[Quasimoto]

Originally Posted by Bill Verburg

It looks to me that both ends can come down

This has been the source of my confusion. To my eye the front of the car looks high, and the rear about right. My plan was to drop the front a bit, but when I did my measurements, it seemed that both ends of the car are lowered...the rear quite a bit.

Something doesn't quite add up, which was the impetus for my posting.

Originally Posted by 76FJ55

Quasimoto, make sure you are loosening the nut side of the adjustment bolts not the bolt side....

Thanks, that's how it looked like it should be done, so I've been trying to break the nuts free. I'm going to PB blaster them a few more times, then add some heat to see if I can get a result. They are being stubborn.

[Bill Verburg]

Originally Posted by Quasimoto

This has been the source of my confusion. To my eye the front of the car looks high, and the rear about right. My plan was to drop the front a bit, but when I did my measurements, it seemed that both ends of the car are lowered...the rear quite a bit.

Something doesn't quite add up, which was the impetus for my posting.



......

It's sort if difficult to measure the heights the factory way, lots of things can throw it off and it is a game of mm.

Here's the range from street to race



and this one has most of the specs the factory has used over the years

[3rd_gear_Ted]

Mr. Bill,
The rear height values that are zero and a negative number sure could use some measurement process definition to make sense???

Wondering about the front heights too from the aspect of the front roll center, race is higher than the street, why ???

[76FJ55]

For those not familiar with the factory method of measuring ride height, here are where the factory measurements are taken.
for the front a larger number is lower. A-x=ride height
for the rear a smaller number is lower. x-A=ride height

[Bill Verburg]

Originally Posted by 3rd_gear_Ted

Mr. Bill,
The rear height values that are zero and a negative number sure could use some measurement process definition to make sense???

Wondering about the front heights too from the aspect of the front roll center, race is higher than the street, why ???

In front a higher # results from lowering the car, 108 is stock 130 is lowered 12mm

In back it's the opposite, lowering the car results in a lower #, if stock is 16mm then 0 is lowered 6mm

here are the factory setup data for early cars, later won't be much different



here are the bump(vorspur or toe change wrt to ride height)




here is the bump curve when the car is lowered ~44mm and a rack spacer is used

[3rd_gear_Ted]

Originally Posted by Bill Verburg

In front a higher # results from lowering the car, 108 is stock 130 is lowered 12mm

In back it's the opposite, lowering the car results in a lower #, if stock is 16mm then 0 is lowered 6mm

here are the factory setup data for early cars, later won't be much different



here are the bump(vorspur or toe change wrt to ride height)




here is the bump curve when the car is lowered ~44mm and a rack spacer is used

Thanks Bill,
The charts help a bunch, RSR torsion bars are about 31/24 ( I think) and the charts correlate those track height settings.

Don't know how much work ($) it is to get the lowered bump curve to be equal like that, great work. I keep looking at 40mm on my ruler and I just can't pull the trigger ($$$)

[Bill Verburg]

Originally Posted by 3rd_gear_Ted

Thanks Bill,
The charts help a bunch, RSR torsion bars are about 31/24 ( I think) and the charts correlate those track height settings.

Don't know how much work ($) it is to get the lowered bump curve to be equal like that, great work. I keep looking at 40mm on my ruler and I just can't pull the trigger ($$$)

That's a stock bump curve, they all have the same one.
T-bas just limit the range of motion.
A rack spacer moves the x axis down by the thickness of the spacer, most of the rack spacers are 11.5mm thick.

a good rule of thumb is to have the tie rod horizontal when at static ride height

when the A-arm points up as you go out to the wheel that's when bump becomes unacceptable
Here's stock


here's lowered, note the steering arm pointing up, a rack spacer or os ends help to correct that